Static limiting device



Nov. 23, 1937. D, G MCCAA sTATlc LIMITING DEVICE Filed DeC. 2, 1935 Bowan; lNsaan) NVNTOR D/V Patented Nov. 23, 1937 PATENT OFFICE STATIC LIMITING DEVICE David G. McCaa, Lancaster, Pa., assignor to Alan N. Mann, Scarsdale, N. Y., as trustee Application December 2, 1935, Serial No. 52,523

10 Claims.

My invention relates broadly to static limiting devices for radio receiving systems. More specifically my invention relates to an arrangement of thermionic tubes in which a resistance load in the anode circuit of a tube is shunted by a rectier whereby the current through the resistance is limited to sharply defined maximum and minimum values.

I am aware of numerous current limiting circuits which depend upon the electronic action of thermionic tubes. In general such circuits depend upon anode current saturation for the maximum limit and cut off of anode current for the minimum limit. The characteristic curve of such arrangements is characterized by the gradual bending of the curve as the limiting values are approached. While this type of limiter has some utility in telegraphic signaling, where distortion is permissible, I nd it unsuitable for telephonie reception. I propose to overcome these dilculties by the present invention in which a gate7 action definitely and sharply limits undesired static.

One of the objects of my invention is in the limitation of the effects of static in a radio receiving system.

Another object is in a thermionic tube circuit in which a resistance load is shunted by a biased rectifier which aids in holding the current flowing through the resistance within predetermined limits. v

A further object is to shunt a resistance load in a thermionic tube by a rectier which consists of the input of a thermionic ampliiier.

A still furtherv object is to shunt a resistance load in the anode by the output of a triode whose grid amplies the control action of the triode.

Additional objects will be apparent from the accompanying specification, drawing, and appended claims.

My invention is illustrated by the accompanying drawing in which Figure I is one embodiment of my invention applied to a radio receiver,

Figure II is a circuit diagram of a modification of Figure I,

Figure III is a circuit diagram of an embodiment in which the loading tube also acts as an amplier or repeater, and,

Figure IV is a graph illustrating the action of the gate circuits embodied in my invention.

In describing the several gures similar reference numerals will indicate similar parts. Iny sofar as the parts and their functions are common or similar, a single description will be given (Cl. Z50-20) to the slider of a potentiometer I5. A biasing 10 battery I1 is connected across the potentiometer. The positive terminal of the battery is grounded. The control grid I9 of a thermionic gate tube 2| is connected to the high potential -terminal of the secondary 9. and blocking capacitors II, I3 is connected to' the cathode 23 which is grounded. The heater 25 is energized by batteries 21 or alternating current.

The anode electrode 29 is connected to a re- 20' sistor 3|, whose resistance may be of the order of 250,000 ohms. The resistor is connected to the positive terminal of a B battery 33, whose voltage may be of the order of 6 to 45 Volts. The

negative terminal of the battery 33 is connected 25 to cathode 23 and ground. The battery may be by-passed by a capacitor 35. The screen grid 3l is connected to a point of suitable positive po-tential on the battery 33. This potential or the potential of the anode may be adjusted by 30A a potentiometer 39 or the like. If a potentiometer is used, a by-pass capacitor 4| may be connected between the slider and cathode.

In Figure I the anode 43 of a thermionic loading tube 45 is connected to the anode 29 of the 35 limiter or gate tube 2|. The cathode 41 of the loading tube 45 is connected to the slider of a potentiometer 4S. A biasing battery 5| is connected across the potentiometer. 'I'he negative terminal of the biasing battery is grounded. The 40 heater 53 may be energized by a battery 55 or the like. The connection of the grid 5I of tube 45 will be described below.

An amplier tube 59 is connected as follows:

The grid electrode 6| is connected to the anode 45 29 of gate tube 2|. The cathode 63 is connected to the positive terminal of biasing battery 65. The negative terminal of this biasing battery is grounded. The heater 6I is energized by a battery 69. through a resistor r'I3 to the positive terminal of B battery I5. The negative terminal of the B battery is grounded.

A second amplifier tube 'I1 is connected to the output of the preceding amplifier 59 as follows: 55v

The secondary 9 is 6 The junction of the Variable l5' The anode electrode 'II is connected 50 The grid electrode 19 is connected to the anode 1| of the preceding tube 59. The cathode 8| is connected to the positive terminal of biasing battery 83. The negative terminal of the biasing battery 83 is grounded. The heater 85 vis energized by a battery 81. The anode electrode 89 is connected through a resistor 9| to the positive terminal of a B battery 93. The negative terminal of the B battery is grounded. The anode 89 is also connected to the grid 51 of the precedl ing loading tube 45. The anode 89 is connected to a capacitor which acts as the output coupling of the tube 11. The several batteries of the preceding tubes may be by-passed by capacitors not shown. Y

The output circuit of the secondamplier tube 11 is connected to a tunable circuit 91 through the coupling capacitor 95 and ground. The tunable circuit is comprised of an inductor 99 which is shunted by a tuning capacitor I0 This tunable circuit is connected to a' suitable radio frequency detector |03, such as a grid detector, or the like. Detectors of this type are so well known to those skilled in the art that a detailed description is not required. The output of the detector may be any suitable signal indicating device, such as headphones |01; The detector output may be amplified and reproduced by va loudspeaker or the like. If greater sensitivity is required one or more stages of tuned radio frequency may precede the detector, or may be coupled between the antenna and the gate tube 2|.

Having described the circuits I shall now explain the operation of the circuit of Figure I. The gate tube has a characteristic represented by the curve XABC of Figure IV. A relatively low voltage is applied to the anode of this tube through the relatively high plate resistor 3|. The sharp angular change of current at A is a characteristic which depends upon symmetrical tube elements, the unipotential heater, and the low electron velocities. The operation of a gate tube of this type is fully explained in my co-pending application Serial No. 6,014 entitled Thermionic tube circuits which was led on February 11th, 1935.

The limitation between AX and the base line of the graph covers a relatively large range of input voltages and furthermore operation on the curved portion C causes distortion of signal currents. In order to act as an eifective limiter for small input voltages and without distortion a small gate must be made. This is accomplished as follows: Consider rst the plate-cathode connection of the load tube 45. The voltage drop in the resistor 3| is opposed to the B battery voltage. The resultant potential of the anode 29 with respect to ground is slightly positive when the anode current in the gate tube 2| is at its maximum value. The cathode 41 is biased slightly more positive than the anode 29. Thus this bias voltage keeps the anode 43 and cathode 41 from conducting when the anode current in the gate tube 2| is a maximum.

When an incoming signal representing voltage drives the grid |9 increasingly negative, less current will flow through the resistor after the point A on the characteristic curve has been passed. As less current flows the positive potential of the anode 29 rises. The bias voltage between the cathode 41 and anode 43 of tube 45 will be exceeded and current will ow through the anode-cathode circuit of tube 45. As the voltage increases the current increases and a constant value is soon reached for the sum of the currents flowing through the resistor 3| and the anodecathode 43-41. This constant current condition is represented by the horizontal portion YB of the curve XABY of Figure IV.

In order that the gate AB may be made sulficiently narrow, it is desirable to supplement the diode action of the loading tube 45 with grid control. Small voltage changes across the resistor 3| are amplified by the thermionic tubes 59-11. The effect of decreasing currents in resistor 3| is to increase the currents in the resistor 13 of tube 59. Increasing currents in the resistor 13, cause a decreased current to flow through resistor 9| of tube 11.

When less current flows through resistor 9| there is less voltage drop in the resistor to oppose the potential of the B battery 93. The potential of the anode 89 of tube 11 becomes more positive TheV grid 51, being connected to the anode 89, becomes more positive and thereby increases the conductivity of tube 45. Thus the voltage changes across resistor 3| are amplified and the amplied changes increase the sharpness of the angle at B and the narrowness of the gate AB. The grid 51 may be initially biased positively or negatively by a battery 58. Generally this bias battery 58 may be omitted.

With the gate AB operating to just admit a signal voltage represented by P, this voltage will be faithfully reproduced as current changes Q in the resistor 3|. A static representing signal R. will be limited as shown by the heavy line S. Thus static voltages are limited to the gate width AB. l

Both types of current changes Q and S are transferred to the detector |03. The detector integrates the signal representing currents, and the static currents. The former is of relatively long duration. The latter is of short duration. After limitation, Vthe static is generally a short, sharp impulse which, when integrated, will have a sound effect which is slight compared to the signals.

In Figure II, the circuit arrangements are somewhat similar to Figure I, but differ therefrom in the manner of connecting the loading tube 20| across the resistor 3|. The grid 203 is connected to the upper terminal of resistor 3|. The cathode 205 is connected to the slider of a potentiometer 201. A biasing battery 209 is connected across the potentiometer. terminal of the biasing battery is grounded.

The anode 2|| is connected to the resistor 3| at a point intermediate its terminals. The anode 2| I is also connected through a coupling capacitor 2 I3 to the high potential terminal of the tuned circuit 91. The action of the loading tube 20| is substantially the same as the loading tube 45 in The negative Figure I. In the Figure II the control action is Y obtained by connecting the grid 203 to the anode In this connection the grid 203 is less positive than the anode 2| with respect to ground. The bias battery 209 can be used to make the grid 203 negative with respect to cathode 205. As the anode 29 of tube 2| becomes more positive with respect to ground, the anode 2| and grid 203 will become positive and makethe loading tube 20| conductive. Thus the action is similar to Figure I. The amount of change of current in resistor 3| which can take place before the loading tube 20| becomes conductive is determined by the bias voltage between cathode 205 and ground. In any event the conductivity of the loading tube 20| acts to limit the minimum current which will owin the resistor 3 I.

In Figure III the loading tube 30I is connected as follows: The grid -303 is connected to the anode 29 and resistor 3l of the preceding gate tube 2|. The cathode 305 is connected to the slider of potentiometer 307. A biasing battery 309 is connected across the potentiometer 301. The negative terminal of the battery is grounded. A bypass capacitor 3I0 is connected between cathode and ground.

The anode 3| I is Connected through a primary 3I3 to the positive terminal of a B battery 3I5. The negative terminal of the B battery 3I5 is grounded, The primary 3I3 is mutually coupled to the secondary 99.

The operation of the circuit of Figure III depends upon the conductive action between grid 303 and cathode 305 of the loading tube 30 I. The cathode is biased by the potentiometer 30'I to prevent the grid 3D3-cathode 305 path from becoming conductive until the voltage drop across the resistor 3I has reached a predetermined value. The anode 3II with its primary 3I3 repeats the voltage variations across the resistor 3l in the input circuit of the loading tube 30 I.

The loading tube 30| thus acts not only as a limiting rectifier but also as a repeater or amplifier tube. The conductive path from grid 303 to cathode 305 is biased to delay its operation until the voltage drop across the resistor 3I exceeds the bias voltage. The operation of the gates of all three circuits is substantially the same and differs only in refinements of operation. These refinements are that small voltages will operate the gate of Figure I to its maximum limits, while the system of Figure II will require higher voltages and Figure III will require still higher voltages.

While I have illustrated the limiting operation of the gate as applied to a relatively simple tuned radio frequency amplifier and detector, it should be understood that I do not limit my invention to any particular species of circuit arrangement. My invention may be applied to the radio frequency amplier or intermediate frequency amplifier of a superheterodyne receiver. Numerous modifications within the scope of my invention will be obvious to those skilled in the art. For example, trodes with or without neutralization,'may be substituted for pentode tubes and vice versa. Likewise other than thermionic tubes may be used as rectifiers. I only intend to limit my invention as required by the prior art and appended claims.

I claim:

1. In a static limiting system, a thermionic tube having a grid, an input circuit including said grid and an output circuit; a resistance connected to said output circuit; means for limiting the maximum current in said output circuit to a sharply defined and substantially constant value in spite of increasing positive voltages applied to said grid; a loading tube effectively connected across said resistance, and means biasing said loading tube so that said loading tube will become conductive as the voltage applied to said grid is made less positive whereby said loading tube will limit the minimum value of current in said output circuit to a substantially constant value in spite of increasing negative voltages applied to said grid.

2. A static limiting system including a thermionic tube having a grid, and an input circuit including said grid, and an output circuit including an anode and a resistor; means for limiting the maximum current in said output circuit to a sharply defined and substantially constant value in spite of increasing positive voltages applied'to said grid;` a triode loading tube having anode, grid, and cathode electrodes, connections from said anode to the junction-'of the first mentioned anode and resistor; means biasing said cathode positively with respect to the second mentioned anode; an amplifier whose input circuit is effectively connected to said resistor and whose output circuit is connected to the second mentioned grid, whereby said triode becomes conductive when said bias is exceeded and said conductivity increases rapidly by the control action of said amplifier on the second mentioned grid.

3. In a device of the character of claim 1, a loading tube in whose current path a grid electrode is included to increase the conductivity of said loading tube by applying a positive charge to said grid as the bias voltage is exceeded.

4. In a device of the character of claim 1, a triode loading tube whose input circuit is connected across said resistance and whose output circuit includes means for repeating the potential variations across said resistance.

5. In a static limiting system, a thermionic tube having a grid, an input circuit including said grid and an output circuit including an anode; a resistance connected to said output circuit; means ior limiting the maximum current in said output circuit to a sharply defined and substantially constant value in spite of increasing positive voltages applied to said grid; a triode loading tube having its grid connected to said anode, its anode connected to a point intermediate the terminals of said resistance; and an output coupling means connected to the output of said loading tube.

6. In a static limiting system, a thermionic tube having a grid, an input circuit including said grid and an output circuit including an anode; a resistance connected to said output circuit; means for limiting the maximum current in said output circuit to a sharply defined and substantially constant value in spite of increasing positive voltages applied to said grid; a triode loading tube having its grid connected to said anode, its anode connected to a point intermediate the terminals of said resistance; means for biasing said loading tube to delay its loading action; and an output coupling means connected to the output of said loading tube.

'7. In a static limiting system, a thermionic tube having grid, cathode and anode electrodes; an input circuit including said grid and cathode, and an output circuit including said anode, a resistor, and said cathode, means for limiting the maximum current in said output circuit to a sharply defined and substantially constant value in spite of increasing positive voltages applied to said grid; a triode loading tube having its input circuit connected across said resistor; and means biasing said input circuit to prevent conduction until a predetermined potential is applied whereby the minimum current iiowing in said first mentioned anode-cathode path is limited to a sharply defined and constant value.

8. In a device of the character of claim 7 coupling means effectively connected to the output circuit of said loading tube.

9. In a static limiting system a thermionic tube having grid, anode and cathode electrodes; an input circuit including said grid and cathode electrodes; an output circuit for said tube effectively connected to said anode and cathode and including twoA parallel paths; means for sharply and definitely limiting Ythe maximum current in one of said paths to a constant value; and means for limiting the minimum currents in both of said paths to a. sharply defined constant value.

10. In a device of the character of claim 9, means for delaying the flow of current in one of said paths until a predetermined potential rise occurs in the other of said paths.

DAVID G. MCCAA. 

